Pinned host memory from a process-wide pool; the compressed chunk uploaded from pinned staging

The GPU engines are built per worker per step, and each took its page-locked host buffers with
cudaHostAlloc / cudaHostRegister and gave them back with cudaFreeHost / cudaHostUnregister. Those
calls map and lock pages under the process's memory-map lock, so all workers queue behind each
other: on the 5080 workstation a myoglobin run made ~3400 of them (6-22 thread-s, depending on the
load), 9gdj ~10400 (124 thread-s); on a four-socket host they cost 20-30 ms each.

- CudaHostPtr takes its memory from a pool: a freed buffer is kept in a power-of-two size class for
  the next engine, up to 512 MiB in all; a buffer above 64 MiB is never kept. Rugnux::Run hands the
  pool back at its end.
- PinnedVector (std::vector with the pool's allocator) replaces the registered vectors of
  AdaptiveSpotFinderGPU and ImagePreprocessorGPU; the Bragg predictors download into a pooled
  pinned buffer and copy into their output, instead of registering it.
- BSLZ4DecoderGPU copies the compressed chunk into a pooled pinned buffer before the upload, so it
  is one DMA instead of the driver's pageable staging (9gdj: 163 GB pageable H2D, 52 s of copy
  time, -> 147 GB pinned, 13 s).

Measured with nsys on the workstation: myob 3398 -> 1396 calls (674 of them the release at the
end), cytc 2986 -> 1270, 9gdj 10428 -> 1748 (124 -> 3.7 thread-s). Output identical (p.hkl md5 on
myob, cytc, 9gdj; CI tier battery).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EizBKhTqrAYago9KJG3dA3

image_analysis/image_preprocessing/ImagePreprocessorGPU.cu

@@ -314,8 +314,7 @@ ImagePreprocessorGPU::ImagePreprocessorGPU(const DiffractionExperiment &experime
       stream(stream),
       copy_image_to_host(copy_image_to_host),
       gpu_stats(1),
-      cpu_stats(1),
-      cpu_stats_reg(cpu_stats) {
+      cpu_stats(1) {
     // Setup mask. The byte-per-pixel form and its checksum come from the PixelMask, which derives them
     // whenever the mask changes: they are the same for every worker, and deriving them walks every
     // pixel of the detector - 18 million of them on a 16 Mpx one, per engine, with an engine built per